This invention relates to a time of flight analysis device and in particular to an inductively coupled plasma time of time of flight mass spectrometer. However, the invention has application to time of flight instruments in which a sample is produced by methods other than ICP techniques, such as MALDI techniques, electro spray or a APCI and elemental analysis by glow discharge or DC plasma means.
Time of flight mass spectrometers analyses a sample by first vaporising and then ionising the sample using a number of possible techniques such as those described above. Once formed, the ionised components of a sample are directed towards an electrostatic accelerator usually by some form of ion optics which collimates or focuses the ion beam. The accelerator imparts a specific kinetic energy to all the ions having the same charge, producing a pulse of ions in which the individual ion velocities are inversely proportional to the square root of the mass to charge ratio. The ion pulse is then directed towards an ion detector a well defined distance away. In travelling to the detector the original ion pulse will be dispersed as a result of the different velocities of the different ion masses. The distribution of ionic masses in the initial pulse and hence the sample can then be determined by measuring the time each ion or group of ions takes to travel the known distance to the detector.
The object of the present invention is to improve existing time of flight analysis devices.
A first aspect of the invention may be said to reside in a time of flight analysis device, including:
means for producing an ionised sample for analysis;
a time of flight cavity;
means for receiving the ionised sample and for directing the ionised sample into the time of flight cavity;
a detector for detecting the ionised sample after travel through the time of flight cavity;
an analysis means coupled to the detector for receiving an output from the detector, said analysis means including;
(a) time to digital conversion circuit means for receiving the output; and
(b) an integrating transient recorder circuit means coupled in parallel with the time to digital conversion circuit means, also for receiving the output from the detector.
By coupling both a time to digital conversion circuit and integrating transient recorder circuit to the detector the dynamic range of the device is greatly improved. The time to digital conversion circuit means can determine results for very small concentrations of sample in which very few ions of the desired species may be present in the ion flow, and the integrating transient recorder circuit means analysing much higher abundant species which may be present to thereby give the increased dynamic range of the device.
Time to Digital Conversion (TDC) uses a system of accurate timers to accurately record the actual arrival time of each pulse induced by arrival of individual ions at the detector. By summing many consecutive spectra a picture of the relative quantities and mass of the constituents is built up. While very fast and accurate, a key limitation of this approach derives from the fact that the system detects ion pulses as logical events (ie. ion present or ion not present). If then two or more ions arrive within one time bin, or if the pulse produced by the detector is induced by arrival of a short multi ion packet, count from only one event is recorded and the other ions are lost. State of the art multiple stop time analysers for TOF MS have now days excellent timing resolution (fractions of nS) and dynamic range, but still suffer from high dead time ( greater than 50 nS). The dynamic range of such a system depends only on the upper counting limit of the counters used, providing the threshold of the discriminator usually installed between analogue and digitising parts eliminates all the analogue noise sources (like preamplifier noise etc.) At a rate of 10-50 kHz (typical repetition frequency of single scan in TOF MS), having  less than 0.5 ions per peak per scan, the dynamic range of 1 e6, achievable in quadruple ICPMS within 1 min of acquisition, would require 7-33 min of acquisition, making xe2x80x9cTDC onlyxe2x80x9d technique not very practical. In the Integrating Transient Recorder (ITR) a transient signal of a single waveform from a repetitive bunch is digitised by a very high speed ADC (typically at less than 5 ns sampling rate), and then data representing the magnitude of signal waveform at each sampling point is temporarily stored in some buffer memory for further summation with the set of data representing the next waveform. After predetermined number of summations integrated data are outputed in the form of magnitude-time array. ITR based techniques always employ some data reduction via summation before storage, as real time spectra acquired at very high repetition frequency of 10-50 kHz can not be stored individually on line. Moreover, ion detectors usually have very high standard deviations on the gain resulting in single ion pulse height distribution with up to 100% RSDs, so ITR can not be used for quantitative analysis when less than a 100 integrated ion pulses represent mass peak. As a result of that and due to limited noise figures of analog signal processing means, the dynamic range of the technique is limited to about 1e4 value. Thus incorporating both a multistop TDC and a Integrating Transient Recorder in parallel, allowing acquisition and processing of TOF mass spectra where any mass peak can contain 1e-5 to 1e3 ions, said limits are given as an example only are defined by ion extraction pulse repetition frequency (100 kHz) and linearity range of ion detector.
Preferably the analysis means also includes a logarithmic preamplifier for extending the dynamic range of the integrating transient recorder circuit means and pulse stretching after a discriminator to increase the effectiveness of the time to digital conversion circuit means.
The second aspect of the invention relates to protection and extension of the lifetime of detectors used in time of flight analysis devices. Modern ion detectors suffer from low dynamic range, being unable to withstand high input currents which may destroy the detector or significantly reduce its lifetime. Discrete dynode electro multipliers have been demonstrated to have higher range of acceptable incoming ion current at which the gain is not declining, in comparison to continuous dynode detectors. However, at extremely high count rates even discrete dynode detectors age quickly. This results in two problems. The overall lifetime of detectors becomes unpractically short and maintenance of a constant gain as the detector wears out requires a change in the voltage applied across the dynodes. The change in voltage is conventionally done by either changing detector entrance DC potential with the anode kept at virtual ground or by applying a DC potential to the anode, which is capacitively decoupled with a preamplifier or the last dynode. Changing the ion detector entrance potentials in time of flight mass spectrometers necessarily implies changing liner voltage. This means the average ion energy changes which means the mass scale has to be recalibrated for every value of detector voltage. Supplying DC potentials to the anode or the last dynode means capacitively coupling DC high voltage supply to the input of a preamplifier so that the AC ripple of the supply affects noise of the detection system.
A second aspect of the invention may be said to reside in a detector for a time of flight mass spectrometer, including:
an ion sensitive surface for receiving a flow of ions;
a plurality of discrete dynodes arranged in series for receiving a secondary emission from the ion sensitive surface when ions impact on the ion sensitive surface, the secondary emission being amplified by the discrete dynodes; and
means for varying the voltage between the ion sensitive surface and an adjacent dynode or between any pair of adjacent dynodes in this series of discrete dynodes so that a voltage between the ion sensitive surface and a last of the dynodes in this series of dynodes can be maintained, substantially.constant and the gain of the detector is varied by varying the voltage between the ion sensitive surface and the adjacent dynode or between said any pair of adjacent dynodes by, the means for varying the voltage.
Thus, as the detector wears through the lifetime of the detector it is not necessary to alter the voltage across the detector and the voltage can be kept at a constant voltage, such as a maximum constant voltage, thereby eliminating the need to recalibrate the mass scale which would otherwise be necessary should the voltage have been changed. In order to change the gain as the detector wears the means for varying the voltage need only be adjusted to change the voltage between the ion sensitive surface and the adjacent dynode or between the pair of adjacent dynodes in the series.
This aspect of the invention may also be said to reside in a detector for a time of flight analysis device, including:
an ion sensitive surface for receiving an ion flow and for producing a secondary emission;
a plurality of discrete dynodes arranged in series with respect to one another for receiving and amplifying the secondary emission; and
means for preventing at least a part of the secondary emission or amplified secondary emission from reaching any one of the dynodes.
Thus, according to this aspect of the invention the detector can be protected from high ion currents which are produced when a high concentration of a particular ion species is present in the sample to be analysed and which otherwise would produce significant wear of the detector should that species be detected by the ion sensitive surface and amplified by the series of dynodes. By preventing the secondary emission resulting from that species from reaching any one of the dynodes the dynodes down stream of that dynode can be protected from the increased secondary emission current which would otherwise be produced to thereby increase the lifetime of the detector.
Preferably the secondary emission or amplified secondary emission is prevented from reaching any particular one of the dynodes by temporarily maintaining that dynode, or a control electrode in the vicinity of the dynode, at a potential different from its normal operating potential thereby deflecting the secondary emission or amplified secondary emission from its normal path.
Preferably a quenching electrode is provided for receiving the secondary emission after the deflection from its normal path so that the secondary emission is effectively removed from the sensitive region of the detector before the various dynodes and electron potentials are restored to the normal operating values.
A third aspect of the invention is related to the push out of packets of ions into the time of flight cavity of an analysis device. Time of flight mass spectrometers which include a means for producing an ion source such as by ICP (inductively coupled plasma) and which include an orthogonal accelerator to move packets of ions transverse to their original direction of travel into a time of flight cavity are known. The time at which the orthogonal accelerator is biased to push the ions in the transverse direction into the time of flight cavity is used as a timing thresholding for the time of travel of the ions from the orthogonal accelerator to the detector to provide the time measurement for the ion travel. Orthogonal acceleration time of flight mass spectrometry has the distinction of a higher duty cycle ideally reaching 50% or higher values.
Another known advantage of TOF MS over other techniques is known to be xe2x80x9csimultaneousnessxe2x80x9d of the technique, meaning that all the ions created at a particular time moment in the ion source are then separated according to their mass to charge ratio within the TOF analyser and form mass peaks of the same mass spectrum. That is, each individual single ion extraction spectrum contains ions created at the same time in the ion source, so that not only processes happening within the ion source may be observed (with typical sampling frequency of up to 100 kHz), noise of the ion source may be almost completely eliminated, giving better precision and better accuracy of isotope ratio measurements. However this is true only if ions created simultaneously in the ion source are translated into the extraction region (ie the orthogonal accelerator) of TOF instrument simultaneously to be sampled by the same extraction pulse. Unfortunately, this is not (and never) the case, as ions are extracted from the source usually by means of an electrostatic field, accelerating ions to a certain predetermined energy. As a result, ions of different masses created simultaneously in the ion source, accelerate to different velocities (depending on square root of mass), and arrive at the orthogonal extraction region at sufficiently different time, so that some ions from the group of ions created simultaneously miss the extraction pulse completely, some ions are extracted, and some ions are extracted by the next oncoming extraction pulse together with the ions created within the ion source at a later time.
The third aspect of the invention may be said to reside in a time of flight analysis device, including:
means for producing a ion beam and for directing the ion beam in a first direction,
a first orthogonal accelerator for directing some of the ions in the ion beam transverse to the first direction into a time of flight cavity;
a second orthogonal accelerator;
an ion mirror for reflecting ions in the beam which pass through the first orthogonal accelerator and the second orthogonal accelerator back in to the second orthogonal accelerator, so the second orthogonal accelerator can push the reflected ions transverse to the first direction into the time of flight cavity; and
a detector for detecting the ions after the ions pass along the time of flight cavity.
According to this aspect of the invention heavy ions such as uranium ions are pushed sideways by the first orthogonal accelerator and lighter ions such as lithium ions which are produced at the same time and at the same temporal position as the uranium ions pass through the first orthogonal accelerator, the second orthogonal accelerator and are reflected by the ion mirror back into the second orthogonal accelerator where they are pushed sideways into the cavity at the same time as the uranium atoms by biasing the first and second orthogonal accelerators at the same time. By reflecting the lighter ions back into the orthogonal accelerator the direction of travel of the lighter ions is such that the two orthogonal accelerators can be arranged symmetrically with respect to the detector so that the path of travel of ions pushed out by both orthogonal accelerators will be identical. Thus, ions produced at the same position at the same time can be pushed out at the time of flight cavity at the same time for detection. Thus, the lighter ions are effectively extracted from a longer section of the beam travelling in the first direction than the heavier ions so that the duty cycle for the lighter ions is improved and becomes comparable to the duty cycle for the heavier ions.
Preferably the first and second orthogonal accelerators are separate from one another and accelerating and focussing means is provided between the first and second orthogonal accelerators. The energy to which the ions are accelerated between the first and second orthogonal accelerators and the dimension of the space between the first and second orthogonal accelerators may, however, be arranged in such a way that light ions which are let through during the extraction cycle of the first orthogonal accelerator (that is when the heavier uranium ions are pushed sideways) enter the second orthogonal accelerator leave it and returning back to the second orthogonal accelerator during the fill time (that is the time at which ions are travelling into the first orthogonal accelerator after the first extraction) of the next extraction cycle. These lighter ions are then pushed out by the second orthogonal accelerator by a push out pulse or bias supplied to both orthogonal accelerators simultaneously.
A further aspect of this invention may also be said to reside in a time of flight analysis device, including:
means for producing an ion beam;
a time of flight cavity;
orthogonal accelerator means for receiving the ion beam and for deflecting the ion beam sideways into the time of flight cavity;
an ion mirror at one end of the time of flight cavity for receiving the deflected ion beam and reflecting the deflected ion beam;
a detector for receiving the reflected ions; and
a second ion mirror arranged transversed with respect to the first ion mirror for reflecting at least some of the reflected ions from the first mirror to the detector.
This aspect of the invention enables a relatively long orthogonal accelerator to be used so that lighter ions which are extracted from the longer section of the orthogonal accelerator undergo a second reflection by the second ion mirror so they are detected by the detector and do not miss the detector. Thus, once again lighter ions which are extracted from the longer section of the beam can be detected with the heavier atoms thereby increasing the duty cycle of the device.
Preferably the second ion mirror extends from the said one end of the time of flight cavity to the detector which is arranged at a first end of the flight cavity adjacent the orthogonal accelerator.
A further aspect of this aspect of the invention relates to duty cycle enhancement of orthogonal acceleration.
In an idealistic situation, when ion energy in the direction along the beam has no spread, all the ions with the same longitudinal energy reach the same point on the target (detector) after travelling through the time of flight analyser, as the trajectories of ions in electrostatic ion optics are energy dependant and mass independent. Usually, during adiabatic expansion through the orifice of the sampler cone of the analyser, ions of all masses pick up same average velocity (that of the bath gas). As a result the average energy is mass dependant and average final coordinate of the ions population when they rich the detector is mass dependent.
In real life, however, ion velocities and energies are defined by a variety of the processes occurring in the ion source and in the interface chamber during the expansion. One of the mechanisms defining ion energy is, for example, capacitive coupling of RF voltage to plasma. The RF potential is distributed long the plasma jet within interface and ions are extracted from the jet from the points (in time and space) which have different electrical potential. As a result, energy of ions is not sharply defined by velocity of the bath gas only, but by the properties of RF plasma coil and RF matching network.
This aspect of the invention may also be said to reside in a time of flight analysis device, including:
means for producing an ionised sample from which a beam of ions is generated;
an orthogonal accelerator for receiving the beam of ions and for deflecting the ion beam sideways;
a detector for detecting the ion beam deflected sideways by the orthogonal accelerator;
the orthogonal accelerator being longer in the direction of the ion beam than the length of the detector such that low energy and low mass ions reflected sideways from the ion beam from one position along the length of the orthogonal accelerator can arrive at said detector and high energy and high mass ions produced at a different position along the length of the orthogonal accelerator and pushed sideways are also received by said detector.
Thus, this aspect of the invention enables a conventionally sized detector to be used and to enhance the duty cycle of the orthogonal accelerator by simply increasing the size of the orthogonal accelerator.
In one preferred embodiment, the length of the orthogonal accelerator may-be approximately 50 mm and the length of the detector approximately 30 mm.
Preferably the length of the orthogonal accelerator is in the order of 1.5 to 3 times the length of the detector and most preferably approximately 2 to 3 times the length of the detector.
A fourth aspect of this invention relates to time of flight mass spectrometers, particularly to the method of time-spatial focussing in time of flight mass spectrometers.
In orthogonal accelerators (or any other beam chopper) ions are initially distributed within a finite beam width. When an acceleration (push-out) pulse is applied, ions appear at different points of created homogenous electric field (formed by a push-out plate and grid). As a result, they are accelerated to different energies. For example, if the distance between the plate and grid is 10 mm, and a push-out voltage applied is 1000 V, then the potential difference between two points separated by 1 mm would be 100 V. If beam width is 5 mm, then ions from outer edges of the beam would be accelerated to energies different by up to 500 eV. The ions closer to the plate, say, would acquire about 1000 eV. The ions 5 mm away from the plate would acquire 500 eV only. After leaving the orthogonal accelerator as a result, the ions of the same mass would have sufficiently different velocities, and would arrive at a detector at sufficiently different times. This implements increase in mass peak temporal width and hence decreases resolution.
Accordingly, a fourth aspect of the invention may be said to reside in a time of flight analysis device, including:
means for producing an ionised sample from which a beam of ions is generated;
an orthogonal accelerator for deflecting the ion beam sideways, the orthogonal accelerator being configured and powered so that ions of the same charge to mass ratio which are moved sideways from the beam of ions and commence sideways movement from different distances within the beam in the direction of sideway movement are time and spatially focused at a focus position, the spatial focussing being performed according to the following conditions for finite spatial spread                                                         ∫                                                s                  0                                -                                  w                  /                  2                                                                              s                  0                                +                                  w                  /                  2                                                      ⁢                                          ∑                                  n                  =                  1                                ∞                            ⁢                              xe2x80x83                            ⁢                                                "LeftBracketingBar"                                                            1                                              n                        !                                                              ⁢                                          xe2x80x83                                        ⁢                                                                  ⅆ                                                                                                           n                                                    ⁢                          T                                                                                            ⅆ                                                  s                          n                                                                                      ⁢                                          xe2x80x83                                        ⁢                                                                  (                                                  δ                          ⁢                                                      xe2x80x83                                                    ⁢                          s                                                )                                            n                                                        "RightBracketingBar"                                ⁢                                  ⅆ                  s                                                              =          0                ,                            (        5        )            
S0 is coordinate of the ion beam
W is the full width of the ion beam; and
a detector for detecting the beam which is deflected sideways by the orthogonal accelerator.
Since ions which are produced at different positions are focused to the same time and spatial focus the resolution of the analysis device is increased.
Preferably the detector is located at the focus position or the focus serves as a virtual ion source for another time of flight analyser, for example mass reflectron.
The orthogonal accelerator is preferably a two or three plate accelerator.
Preferably for three plate and therefore three stage focussing the spatial focussing is performed according to the following conditions
D=2xc2x7{((s0+xcex94s)xc2xdxe2x88x92(s0xe2x88x92
xcex94s)xc2xd)xc2x7Esxe2x88x92xc2xd+Edxe2x88x921xc2x7(B+
xc2xdxe2x88x92C+xc2xdxe2x88x92Bxe2x88x92xc2xd+Cxe2x88x92xc2xd)+
+Eexe2x88x921xc2x7(A+xc2xdxe2x88x92B+xc2xdxe2x88x92Axe2x88x92
xc2xd+Bxe2x88x92xc2xd)}xc2x7(Axe2x88x92xe2x88x92xc2xdxe2x88x92A+
xe2x88x92xc2xd)xe2x88x921,xe2x80x83xe2x80x83(7)
where
A=sEs+dEd+eEe; B=sEs+dEd; C=sEs;
indexes xe2x88x92, + mean that value of correspondent parameter A, B or C is considered at S=S0xe2x88x92W/2, S=S0+W/2 respectively,
D is distance to spatial focus from exit of orthogonal accelerator
e is gap width of third gap of the 3-step acceleration
d is gap width of second gap of the three-step acceleration
Es, Ed and Ee are the field strengths of the first, second and third stages of the three stage acceleration respectively.
A further problem which occurs with conventional time of flight mass spectrometers is due to significant Coulomb forces which exist at focal points of the ion beam as the ion beam travels from the ion source to the detector. Most time of flight mass analysis employ ion beams at relatively low intensity (less than 1nA). Typical ion currents detected in ICP mass spectrometers are of the order of 10 to 50 nA, with ion energy of the order of 10 eV.
This means very severe space charge effects are happening in low voltage parts of the ion optics especially at the focal points where ions experience significant coolant forces.
The object of a fifth aspect of the present invention is concerned with overcoming space charge-effects, effecting resolution and sensitivity of time of flight analysis devices where ion beams are focussed to a small point so that a large number of ions may be present in a very small area of space where space charge effects may effect the resolution and sensitivity of the instrument.
This aspect of the invention may be said to reside in a time of flight analysis device, including:
means for producing an ionised sample from which a beam of ions is generated;
an orthogonal accelerator for deflecting the ion beam sideways;
beam forming optics between the means for producing the ionised sample and the orthogonal accelerator for focussing the beam of ions so that at every focus plane the beam is focussed such that one dimension of the beam is larger than another dimension of the beam;
a detector for detecting the ion beam deflected sideways by the orthogonal accelerator; and
vertical focussing means between the orthogonal accelerator and the detector for focussing the beam back to a size commensurate with the size of the detector.
Preferably the beam forming optics focus the beams at every focus plane between the means for producing the ionised sample and the orthogonal accelerator, the beam has a dimension of about 30 mm by 3 mm.
Preferably the vertical focussing means is located at a position where ions of different masses are separated in time so that space charge effects are less severe when the beam crossover becomes smaller after vertical focussing.
This aspect of the invention may also be said to reside in a time of flight analysis device, including:
means for producing an ionised sample from which a beams of ions is generated;
an orthogonal accelerator for deflecting the ion beam sideways for producing ion packets, the ions in each packet separating as the ions in each packet move sideways due to different mass charge ratios of the ions in each packet; and
vertical focusing means located at a position where the ions in the ion packet have separated in time, for vertically focusing the ions which have been separated so as to avoid excessive space charge effects.
A sixth aspect of the invention concerns pump size for producing vacuums within the time of flight analysis device.
This aspect of the invention may be said to reside in a time of flight analysis device, including:.
an interface chamber for receiving an ion beam;
a main pump for evacuating the interface chamber;
an intermediate chamber for receiving the ion beam from the interface chamber;
a first low pressure pump for evacuating the immediate vacuum chamber, the first pump being coupled to the main pump;
a main vacuum chamber for forming a time of flight cavity and for receiving ions for time of flight travel to a detector;
a second low pressure pump for evacuating the main vacuum chamber;
an additional chamber between the intermediate chamber and the main vacuum chamber;
a third low pressure pump for evacuating the additional chamber, the third low pressure pump being coupled to the main pump; and
the second low pressure pump being coupled to the third low pressure pump.
By the inclusion of the additional vacuum chamber and the third low pressure pump the pump size of the low pressure pumps and main pump can be reduced and although an additional pump is required. Pump costs are reduced in view of the ability to reduce the size of the pumps.
Preferably a petition wall is arranged between the main vacuum chamber and the additional vacuum chamber.